Unidirectional material

Fig. 2. Three layers of unidirectional material stacked together to form a laminate.

Additional factors which must be taken into account are environmental effects (thermal as well as chemical), effects of defects, statistical variability of the material, long-term behavior, and cyclic versus static loading effects. Assessment of these effects requires the end user to conduct a large series of tests using multiple specimens. A typical series will examine a unidirectional material in tension in the 0, 90, and cross-ply directions 0, 90, and cross-ply in compression and 1-2, 1-3, and 2-3 shear at different temperatures ranging from —54°C to the expected service temperature creep rupture at temperatures up to the expected service temperature and fatigue at room and elevated temperature. This series of tests, shown in Table 12.1, may require over 400 specimens. 

Fara, S. and Pavan, A. Fracture toughness in short fibre composites analysis of fracture mechanisms in relation to fibre orientation in unidirectional materials to be submitted. 

If, as in the fracture surface type shown in Fig. 3 on the left hand side, the width of the featureless delamination is increasing with delamination length this is also reflected in the R-curve (Fig. 5). There, values are dropping from relatively high cross-ply -values to a value typical of the unidirectional laminate. This could be a hint that energy release rates determined for the cross-ply laminates using the analysis developed for the unidirectional material are valid in the sense that relative proportions are conserved. 

The different bending stiffness of the unidirectional, symmetric, non-symmetric lay-up is shown by independent three-point bending E-modulus measurements to decrease in that order from about 145 GPa to 73 GPa and 62 GPa. The same trend, with somewhat larger values i > seen in the back-calculated E-moduli (Tables 2-4). This is also apparent from the larger displacements (at similar delamination lengths) of the cross-ply lay-ups compared with th- unidirectional material. 

There is a considerable difference between initiation values from the insert starter film determined from the maximum load/5% offset (MAX/5%) and the non-linear or visual point (NLWIS) for the two cross-ply laminates (NLA IS-values not reported in the Tables since they were not determined for all specimens). The NLATS-values, when determined, are comparable to initiation from the insert for the unidirectional material (determined as MAX/5%- or NL/VIS-values). The first few mm of delamination propagation in the cross-ply laminates usually occur in the mid-plane of the beam, but deviating from it before the maximum load value is reached. The difference between the NL/VIS- and MAX/5%-values quite likely reflects this transition from mid-plane to saw-tooth pattern. 

Wear testing, when conducted, would follow standard procedures, while being aware of the anisotropic nature of the material under test. For example, for a unidirectional material, wear along the fibers would be different from across the ends of the fibers. 

Pipelines are typically operated with a unidirectional material flow. In typical settings the associated partners along the pipeline can be categorized distinctly in (pure)... 

Most structural composites are highly anisotropic, consisting of unidirectional materials where the mechanical properties in the longitudinal (or 0°) direction are fibre dominated, while in the transverse (or 90°) direction they are matrix dominated. 

Most carbon fibre reinforced plastics (CFRP) used and investigated to date are produced from preimpregnated continuous carbon fibre prepregs. Polymers reinforced with aligned short carbon fibres have certain advantages as materials for structural components, because they can easily be formed into complicated shapes with satisfactory mechanical properties. Woven fabrics produced from carbon fibres find increasing application in the aerospace and many other industries, because they are easy to handle, they have the ability to conform to complicated shapes and the in-plane properties are more isotropic than those of equivalent unidirectional materials. 

Strength calculations on angle-ply laminates are based upon the elastic anafysis described in Section 8.4.2. llie strains produced in the laminate by a given set of applied stresses are first calculated, using the computed laminate stiffigess constants. Stresses corresponding to these strains are then calculated for each layer of the laminate. These stresses are then expressed in terms of stresses parallel to and normal to the fibres, and the combination of stresses is compared with strerigth criteria for unidirectional material, which should preferabfy be obtained by experiment... 

Bi-directional woven construction laminates do not perform as well as unidirectional materials. The presence of crimp in the fibres gives rise to a local stress concentration in the polymer which may cause cracking and allows the fibre to straighten out under stress. Non-woven continuous fibre reinforcements are preferred in critical applications. The least resistant constructions to creep are the random discontinuous short fibre reinforcements, as they contain a multitude of fibre end stress concentrations and higher shear stresses developed in the polymer to couple between fibres. 

Impact tests are carried out at a fast rate of loading to promote a brittle failure. Izod and Charpy tests are used for isotropic and laminated materials and are not suitable for unidirectional material, or out-of-plane measurements with laminated materials, since notches are not always effective. 

An advanced composite laminate can be tailored so that the directional dependence of strength and stiffness matches that of the loading environment. To do this, layers of unidirectional materials are oriented to satisfy the loading requirements. This allows for almost infinite variations in properties to cover individual needs. 

Sims et al. [197] have pointed out that a redraft of ISO 527 tensile testing for plastics has resulted in two complementary parts for composites Part 4 based on ISO 3268 [199] covers isotropic and orthotropic materials, and Part 5 covers unidirectional materials. There is a need to harmonize these two methods into a new Part 5. Inputs were included from ISO, ASTM, CRAG, and EN aerospace methods. The remaining three parts are Part 1—general principles. Part 2—plastics and molding materials (including short fiber-reinforced materials), and Part 3—films. ISO DIS 527-5 has in parallel been voted as a CEN standard (EN 527-5) and should, with Part 4, replace EN61 [212] and possibly two aerospace EN standards [34,42,132,160,163-165,199-213]. 

G.D. Sims and A. Vanas, Round-robin on fatigue test methods for polymer matrix composites. Part I, Tensile and flexural tests of unidirectional material, NPL Report DMM(A)180, 1989. 

The experimental longitudinal (fiber direction) mechanical properties of epoxy matrices and E-glass reinforced unidirectional materials (8) are compared in Table 13.9. 

Figure 3 shows stiffness reduction in a [0/90]s SiC/CAS laminate with transverse cracks in the 90° layer without any damage in the 0° plies. The properties of a unidirectional material used in the calculations are again taken from (Daniel and Ishai, 1994). In Fig. 3, stiffness properties of the damaged laminate are normalised by their values for the intact laminate and plotted as a function of transverse crack density C2, which varies from 0 (no damage) to 50 cracks/cm. The maximum crack... 

Figure 3.7 Examples of mat fabrics and a unidirectional material. (Reproduced with permission from Quinn, J.A. (1988), Design Manual of Engineered Composite Profiles, Fibreforce.)...

This chapter is concerned with the short-term mechanical properties — moduli and strengths — of glass, aramid and carbon fibres in a thermosetting resin matrix. A little information on reinforced thermoplastic matrix systems is also included. The data mainly refer to the room temperature properties of 55-65 v/o fibre, unidirectional, systems. The effects of the variation in fibre volume loading, method of test and instantaneous and long term exposure to temperature are briefly mentioned. Longitudinal properties tend to be fibre dominated, and so are compressive properties to some extent for glass and carbon fibres. The anisotropy of unidirectional materials is noticeable. 

The carbon fibres used are from the separate sources and cover all the grades listed in Table 3.1. Measurements were made at room temperature and all the matrices except one are epoxies. The use of bismaleimide resin makes little difference to properties apart from the relatively low transverse strain-to-failure. The large gaps in information especially for transverse, compressive and shear properties are clear. The anisotropy of unidirectional materials can be seen by comparing longitudinal and transverse tensile or shear moduli and strengths. 

The impact and fracture behaviour of composites is complex. Thermosets do not undergo plastic deformation, composites are not necessarily described by linear elastic fracture mechanics (LEFM) and it is very difficult, if not impossible at the present time, to predict the initiation and progression of failure in complicated structures from the behaviour of unidirectional materials. Because of the latter much of the work on fracture has involved studies of plied or laminated specimens. 

The first six results in Table 6.1 are for an impact velocity of 2.44 m/s. The impact energy or toughness depends on the mode of stressing, the presence or otherwise of a notch, Vf, and the material disposition. Particularly noticeable are the low results for the 20 v/o mat specimen and the transverse unidirectional material, where there is no reinforcement to increase toughness. 

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